There is increasing interest in the development of optical sensors, particularly for medical use in the determination of clinically important species. Sensors for sodium, potassium and lithium have proven particularly difficult to develop, in view of the limited complexation of these ions with known chromogenic ligands. Some success has been reported in recent years with chromogenic crown ethers, spherands and cryptands (1, 2, 14). It has been demonstrated that attachment of ionisable chromogenic groups in positions adjacent to the polar cavity of these molecules can produce materials which show striking changes in absorbtion on complexation. Incorporation of a metal cation into the cavity is accompanied by deprotonation and it is the latter process that produces the optical reponse.
Certain calixaryl compounds have been shown to be efficient ionophores for alkali metal cations and have been used to produce ion-selective electrodes for sodium, potassium and caesium (7,8). Chromogenic calixarenes have also been described: Shimizu et. al (3) described an ion-selective chromogenic calixarene of the formula I: ##STR2## This derivative has within the molecule both the triester moiety as a metal-binding site and the azophenol moiety as a colouration site. It was described as having good lithium selectivity. Nakamoto et. al (11) described an azocalixarene of the formula II ##STR3##
This compound showed a lithium specific colour change. King et. al. (19) described chromoionophores of the formula IIa. ##STR4##
These compounds showed a potassium specific colour change.
However the azocalixarenes of formula II and II a having the azo groups directly attached to a calixarene aryl group are relatively difficult to synthesise and require long preparation routes with relatively low yields.
Fluorescence spectrophotometric determination has also been investigated for metal ion analysis. Fluoro-ionophores of cyclic and non-cyclic polyethers (4,5), and, more recently, of a calix(4)arene (6), have been synthesized and reported to produce a marked increase in fluorescence in the presence of lithium ions in the case of the former, and sodium ions in the latter.
Aoki et. al (12) described a fluorescent calixarene of the formula III ##STR5## However this is not a chromoionophore.
Calixarenes having chromogenic moieties have also been described since the priority date of this application by Kubo et al (18).
There is also a need for a non-instrumental detector of volatile amines such as trimethylamine (TMA). TMA is a degradation product of the reaction of bacteria such as Pseudomonas upon trimethylamine oxide in marine fish after death.sup.20. Its detection along with other amines, has been used as a means of determining fish freshness. Traditionally fish freshness has been assessed by olfactory analysis.sup.21 but this is both time consuming and expensive. Colorimetric methods have also been employed and developed successfully, and can distinguish between TMA and dimethyl amine (DMA).sup.22-24, both of which are generated (along with other volatile amines and sulphides) as the fish spoils.sup.20. However, these methods require time-consuming mincing of the fish followed by solvent extraction before analysis. More recently Gastec detector tubes containing crystals which change colour as they react with a specific gas or vapour have been developed and used in amine analysis of the gill air of fish.sup.25 with amines being determined in a concentration range of 0.05 to 5 ppm. These tubes are attached to a pump and a specific volume of gas is analysed. GC anaylsis of amines produced by fish has also been used to distinguish between TMA and DMA and to quantify the levels at which each are present.sup.26. Another approach investigated for TMA analysis involves the use of semiconductor gas sensors containing ruthenium. Such sensors were found to respond well to 50 ppm TMA and were used to determine the freshness of Japanese saurel.sup.27-28. All of these methods, while not all destructive, do involve a certain degree of handling of the samples or involve some form of instrumentation. The development of a non-instrumental indicator system which would respond quickly to gaseous amines could obviously be of benefit to the food industry.
Nakashima et al (29) described a crowned 2, 4-dinitrophenylazophenol which, when complexed with Ba.sup.2+, could be used for detection of volatile amines such as trimethylamine in a flow-injection analysis (FIA system).